Minnaert 1941 paper on the 'reciprocity principle' and other symmetry-considerations:
Photometry of the bright side of the Moon has been carrie dout for at least 100 years. An important paper is the one by van den Bergh:
He observed the Moon at 5 different occassions - two of them at Full Moona nd one of them at half Moon, and derived B-V colours for 12 regions, measured relative to Mare Serenitatis. He also observed standard stars and tied photometry of Mare S. to these - thus tying the B-V photometry of the 12 regions to the standards.
Here are some results and insights:
For Mare S. he found B-V = 0.876 +/- 0.022. Over the 12 regions on the Moon (20'' to 40'' large) he reports B-V relative to M. Ser. This offset has mean value -0.017 +/- 0.005, where the mean was weighted (by me) with the square of the mean error reported by vdB . Knowing B-V for M. S. we can say that an average over 11 regions on the Moon (I dropped one measurement as it was flagged problematic) is 0.859 +/- 0.023.
I would say that this is what vdB62 reports. From somewhere we havethe notion that he reports 0.92 for B-V - I do not see that in his paper. But he does quote Harris for saying that the mean over many regions on the Moon is 0.92, but does not comment further on that.
I notice that vdB does not allow for the different phases of his observations, and I think I know why - the photometry may depend on the lunar phase, but the colour does not (at least for these phases). We should check that, as we have BS B and V images galore!
Another paper, that by Wildey and Pohn: